The present invention relates to a method and apparatus for increasing the lifetime of low pressure Hg/Rare Gas capacitively coupled rf discharge lamps. More, specifically, the present invention relates to a method and lamp which inhibits Hg and rare gas loss in the lamp vessel.
Electrodeless low pressure Hg/Ar discharge lamps offer a number of significant advantages over conventional fluorescent lamps. They require no electrodes thus eliminating electrode failure and resulting in a long lasting lamp. Potentially, they have reduced electrode losses and thus can be highly efficacious. In addition, they remove some constraints on lamp geometry, they may use chemically reactive constituents and they can be made cheaply.
Electrodeless low pressure discharges can be generally categorized as inductive, capacitive, or surface wave coupled. The invention introduced here is concerned with capacitively coupled low pressure discharge lamps. Capacitively coupled lamps have been demonstrated in U.S. Pat. No. 4,266,166 using a 915 MHz rf source. Typically these lamps have all the advantages listed above however they require a high frequency power source which is relatively expensive. To reduce cost of the power source capacitively coupled discharges have been operated at lower driving frequencies. However, the lifetime of such lamps was unacceptable.
When lamps are capacitively driven at frequencies below 100 MHz they possess the same advantages as mentioned above, however, they may have a fairly short life span. After a few days of continuous operation at discharge currents (and light output) comparable to conventionally (electronic or magnetic ballast) driven fluorescent lamps, the Hg in the lamp vanishes and subsequently the argon buffer gas disappears until there is virtually nothing left in the lamp volume and the discharge ceases. When studied under postmortem analysis these lamps have dark patches where much of the Hg in the lamp volume has imbedded itself in the phosphor and in the glass envelope; eventually the argon buffer gas suffers the same fate. In the areas where the Hg is lost, the phosphor in the lamp and the glass underneath it is generally brown or black. The areas of Hg loss are always where the electrodes are connected to the lamp body and where capacitive coupling is applied. Apparently, the ions of Hg (and later, argon) are accelerated by the dc potential between the plasma and the discharge vessel surface, they impact on the non-conducting glass surface and they are permanently lost from the discharge volume. This same phenomena is avoided at higher frequencies (i.e. microwave frequencies) because at these frequencies the sheath voltages are significantly lower resulting in ion bombardment energy that is insufficient to imbed the ion into the phosphor or glass substrate.
The present invention avoids this problem in a unique and novel manner.